This invention relates to an organic electroluminescence element and a display, and particularly to an organic electroluminescence element excellent in luminance of emitted light and lifetime and a display comprising the organic electroluminescence element.
As an emission type electronic displaying device, there is an electroluminescence device (ELD). As elements constituting the ELD, there is an inorganic electroluminescence (hereinafter referred to also as inorganic EL) element or an organic electroluminescence (hereinafter referred to also as organic EL) element. The inorganic electroluminescence element has been used for a plane-shaped light source, but a high voltage alternating current has been required to drive the element. An organic electroluminescence element has a structure in which a light emission layer containing a light emission compound is arranged between a cathode and an anode, and an electron and a hole were injected into the light emission layer and recombined to form an exciton. The element emits light, utilizing light (fluorescent light or phosphorescent light) generated by inactivation of the exciton, and the element can emit light by applying a relatively low voltage of from several volts to several decade volts. The element has a wide viewing angle and a high visuality since the element is of self light emission type. Further, the element is a thin, complete solid element, and therefore, the element is noted from the viewpoint of space saving and portability.
However, an organic EL element for practical use is required which efficiently emits light with high luminance at a lower power.
In U.S. Pat. No. 3,093,796, there is disclosed an element in which stilbene derivatives, distyrylarylene derivatives or tristyrylarylene derivatives are doped with a slight amount of a fluorescent compound.
An element is known which comprises an organic light emission layer containing an 8-hydroxyquinoline aluminum complex as a host compound doped with a slight amount of a fluorescent compound (Japanese Patent O.P.I. Publication No. 63-264692), and an element is known which comprises an organic light emission layer containing an 8-hydroxyquinoline aluminum complex as a host compound doped with a quinacridone type dye (Japanese Patent O.P.I. Publication No. 3-255190).
When light emitted through excited singlet state is used, the upper limit of the external quantum efficiency (xcex7ext) is considered to be at most 5%, as the generation ratio of singlet excited species to triplet excited species is 1:3, that is, the generation probability of excited species capable of emitting light is 25%, and further, external light emission efficiency is 20%. Since an organic EL element, employing phosphorescence through the excited triplet, was reported by Prinston University (M. A. Baldo et al., Nature, 395, 17, p. 151-154 (1998)), study on materials emitting phosphorescence at room temperature has been actively made. (M. A. Baldo et al., Nature, 403, 17, p. 750-753 (2000), and U.S. Pat. No. 6,097,147 etc.) As the upper limit of the internal quantum efficiency of the excited triplet is 100%, the light emission efficiency of the exited triplet is theoretically four times that of the excited singlet. Accordingly, light emission employing the excited triplet exhibits the same performance as a cold cathode tube, and can be applied to illumination.
Several proposals with respect to the phosphorescent material were made in xe2x80x9cThe 10th International Workshop On Inorganic and Organic Electroluminescence (EL ""00, Hamamatsu). For example, Ikai et al. use, a hole transport compound as a host compound of a phosphorescent compound, M. E. Tompson et al. use, as a host compound of a phosphorescent compound, various kinds of electron transporting compounds doped with a new iridium complex, and Tsutsui et al. obtain high luminance due to introduction of a hole blocking layer (an exciton blocking layer). Pioneer Corporation proposes a hole blocking layer employing a certain kind of an aluminum complex. Ikai et al. disclose a hole blocking layer (an exciton blocking layer) employing a fluorine-containing compound in Appl. Phys. Lett., 79, 156 (2001), whereby high light emission efficiency is obtained.
However, the techniques described above do not provide an organic EL element providing both high emission luminance and long emission lifetime.
An object of the present invention is to provide an organic electroluminescence element improved both in emission luminance lifetime and emission lifetime, and a display employing the organic electroluminescence element, which emits light with high emission luminance at reduced power.